Hyperactivity in the muscles of mastication has been causally implicated in a number of adverse dental conditions including the myofascial pain-dysfunction (MPD) syndrome. However, the origins of masticatory muscle hyperactivity per se are poorly understood. Recently multifactorial theories have posited that masticatory muscle hyperactivity mirrors the cumulative or interacting effects of psychobiologic stress and occlusal disharmony. Heretofore, the multifactorial views have been completely untested experimentally. Therefore, this experiment will evaluate the interacting as well as the unilateral effects of a stress manipulation and an occlusal-guidance manipulation on bilateral masseter EMG activity among normal human subjects. In accordance with completely counterbalanced experimental design, 8 females presenting with no recent history or clinical signs of oral-motor disorders will, at a weekly interval, be twice subjected to horrific and idyllic movie segments, once having worn an occlusal interference splint for 19 hr and once having worn a non-interference (placebo) splint for 19 hr. During both experimental sessions bilateral masseteric EMG levels will be recorded as will skin-conductance levels and skin-conductance responses. After both sessions, semantic differential ratings of the movie segments will also be acquired. A 2 x 2 x 2 x 9 (type of splint x movie content x side of recording x interval of recording) ANOVA with repeated measures on all factors will be conducted within masseter EMG change from baseline data and within baseline-independ EMG mean scores. Both ANOVAs make possible the occurrence of main effects and interactions involving the movie content and occlusal disharmony variables. The electrodermal and semantic differential data also will be examined to corroborate the differential stressfulness of the horrific versus the idyllic films. These analyses will make possible the argument that stress was varied successfully via the film-content manipulation.